Jan 18, 2021 |
(Nanowerk News) Why is studying spin properties of one-dimensional quantum nanowires important?
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Quantum nanowires–which have length but no width or height–provide a unique environment for the formation and detection of a quasiparticle known as a Majorana zero mode.
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A new UNSW-led study (Nature Communications, “New signatures of the spin gap in quantum point contacts”) overcomes previous difficulty detecting the Majorana zero mode, and produces a significant improvement in device reproducibility.
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Quantum point contact structure (left) in which an applied voltage constricts electron movement to one dimension, with conductance (right) showing effect of applied magnetic field (red).
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Potential applications for Majorana zero modes include fault-resistant topological quantum computers, and topological superconductivity.
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Majorana fermions in 1D wires
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A Majorana fermion is a composite particle that is its own antiparticle.
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Such unusual particle’s interest academically and commercially comes from their potential use in a topological quantum computer, predicted to be immune to the decoherence that randomises the precious quantum information.
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Majorana zero modes can be created in quantum wires made from special materials in which there is a strong coupling between their electrical and magnetic properties.
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In particular, Majorana zero modes can be created in one-dimensional semiconductors (such as semiconductor nanowires) when coupled with a superconductor.
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In a one-dimensional nanowire, whose dimensions perpendicular to length are small enough not to allow any movement of subatomic particles, quantum effects predominate.
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New method for detecting necessary spin-orbit gap
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One-dimensional semiconductor systems with strong spin-orbit interaction are attracting great attention due to potential applications in topological quantum computing.
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The magnetic ‘spin’ of an electron is like a little bar magnet, whose orientation can be set with an applied magnetic field.
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In materials with a ‘spin-orbit interaction’ the spin of an electron is determined by the direction of motion, even at zero magnetic field. This allows for all electrical manipulation of magnetic quantum properties.
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Applying a magnetic field to such a system can open an energy gap such that forward -moving electrons all have the same spin polarisation, and backward-moving electrons have the opposite polarisation. This ‘spin-gap’ is a pre-requisite for the formation of Majorana zero modes.
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Despite intense experimental work, it has proven extremely difficult to unambiguously detect this spin-gap in semiconductor nanowires, since the spin-gap’s characteristic signature (a dip in its conductance plateau when a magnetic field is applied) is very hard to distinguish from unavoidable the background disorder in nanowires.
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The new study finds a new, unambiguous signature for the spin-orbit gap that is impervious to the disorder effects plaguing previous studies.
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“This signature will become the de-facto standard for detecting spin-gaps in the future,” says lead author Dr Karina Hudson.
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Reproducibility
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The use of Majorana zero modes in a scalable quantum computer faces an additional challenge due to the random disorder and imperfections in the self-assembled nanowires that host the MZM.
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It has previously been almost impossible to fabricate reproducible devices, with only about 10% of devices functioning within desired parameters.
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The latest UNSW results show a significant improvement, with reproducible results across six devices based on three different starting wafers.
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“This work opens a new route to making completely reproducible devices,” says corresponding author Prof Alex Hamilton UNSW).
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Source: https://www.nanowerk.com/nanotechnology-news2/newsid=57035.php